No, the objective isn’t to make assignments optional, but two benefits accrue when students are given some choice about assignments. The first is motivational—when students select the method they will use to master the material, they can pick an option they think they’d like to complete. And if an assignment option looks appealing, that increases the chance that students will spend more time working on it and more learning can then result. Second, the practice confronts students with themselves as learners. With teacher guidance, they can be challenged to consider why they find some assignments preferable. They can be encouraged to consider what skills the assignment involves and whether those are skills they have or need to work on developing. A strategy such as this moves students in the direction of autonomy and maturity as learners.
I had dinner with a group of faculty recently during which we had a prolonged and intense discussion of rubrics—I know, only college teachers could become impassioned about a topic like this. The debate centered on whether rubrics could capture all the aspects of an assignments or whether they constrained both instructors and students.